One reason this can happen is if the quantity of items available is greater than 1, when the auction ends (for example when someone purchases one of the items), the item will be automatically relisted with an end date 7 days from now. This code can be found in /components/com_bfauction_plus/controller.php, around line 1283

// set end date to be 7 days from now
$date = JFactory::getDate();
$date->modify("+7 days");